1. Field of the Invention
The present invention relates to carbazole compounds. Further, the present invention relates to materials for light-emitting elements, organic semiconductor materials, and light-emitting elements using the carbazole compounds.
2. Description of the Related Art
A display device using a light-emitting element (organic EL element) in which an organic compound is used as a light-emitting substance has been developed rapidly as a next generation lighting device or display device because it has advantages that such a light-emitting element can be formed to be thin and lightweight, has very high response speed for input signals, and has low power consumption.
In an organic EL element, when voltage is applied between a pair of electrodes with a light-emitting layer interposed therebetween, electrons and holes injected from the electrodes are recombined to form an excited state, and when the excited state returns to a ground state, light is emitted. A wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance; thus, by using different types of organic compounds as light-emitting substances, light-emitting elements which exhibit various wavelengths, i.e., various colors can be obtained.
For a display device which is expected to display images, such as a display, at least three colors of light having red, green, and blue wavelengths are used in order to show an image with reproduced full-color. For a lighting device, in order to obtain high color rendering property, light having wavelength components thoroughly in the visible light region is ideally obtained. Actually, two or more kinds of light having different wavelengths are mixed to be used in many lighting devices. Note that it is known that by mixing light of three colors, red, green, and blue, white light emission having high color rendering property can be obtained.
As described above, light emitted from a light-emitting substance is peculiar to the substance. However, important performances as a light-emitting element, such as lifetime or power consumption, depend not only on a light-emitting substance but also greatly on layers other than a light-emitting layer, an element structure, properties of the light-emitting substance and a host, compatibility between them, or the like. Therefore, there is no doubt that many kinds of materials for light-emitting elements are needed for the growth in this field. For these reasons, materials for light-emitting elements which have a variety of molecular structures have been proposed (for example, see Patent Document 1).
In particular, a material in contact with a light-emitting material (such as a light-emitting layer or a carrier-transport layer in contact with a light-emitting layer) should be a material having a suitable HOMO level, a suitable LUMO level, a suitable band gap between the HOMO level and the LUMO level, a suitable S1 level, and/or a suitable T1 level for efficient light emission of the light-emitting material. When a material exhibiting light with a shorter wavelength is used, a material with a low molecular weight is generally selected for keeping a small conjugation, as a material in contact with the material exhibiting light with a shorter wavelength. However, when a material with a low molecular weight is used, a structure to be selected is limited. In particular, since a T1 level is lower than an S1 level, a material in contact with a phosphorescent light-emitting material is limited. Therefore, a more preferable material has been expected to be developed.